INTERACTION BETWEEN HALOTHANE AND MU, DELTA AND KAPPA OPIOID AGONISTS ON THE ISOLATED RIGHT ATRIA OF THE RAT

We have examined the interaction between halo-thane and specificopioid agonists at mu (morphine and [D-ala² N-mephe⁴, gly-ol⁵J-enkephalin(DAGO)), delta ([D-pen^{2 5}]-enkephalin (DPDPE)) and kappa (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolikynyl)cyclohexyl]-bencetamidemethanesul-phonate (U-50,488H))receptors on the isolated right atria of the rat. All the opioidagonists tested decreased atrial rate. The maximal effects obtainedwith U-50A88H (75 (SE 3.3)%) were significantly (P < 0.001)greater than those obtained with morphine (12(2.7)<), DAGO(8(0.6)<) or DPDPE (11 (1:8)<). Halothane 1.5 v/% didnot modify the inhibitory effects induced by morphine, DAGOor DPDPE. However, U-50,488H had a potentiating effect in thepresence of halothane 1.5 v/v% (P<0.001). Naloxone 5 x 1O^–7and 1 x 1O^–6 mol litre^–1 antagonized the inhibitoryeffects of U-50,488H in the presence of halothane. We concludethat halothane increased the potency of a kappa agonist on isolatedright atria and suggest that this effect was meded by opioidreceptors. (Br. J. Anaesth. 1992; 69:487–491)